Semiconductor device with junction termination zone

ABSTRACT

A semiconductor device includes a drift zone formed in a semiconductor portion. In a transition section of the semiconductor portion a vertical extension of the semiconductor portion decreases from a first vertical extension to a second vertical extension. A junction termination zone of a conductivity type complementary to a conductivity type of the drift zone is formed between a first surface of the semiconductor portion and the drift zone and includes a tapering portion in the transition section. In the tapering portion a vertical extension of the junction termination zone decreases from a maximum vertical extension to zero within a lateral width of at least twice the maximum vertical extension.

BACKGROUND

Vertical power semiconductor devices control a load current flow betweena first load electrode at a front side and a second load electrode on aback of a semiconductor die. In the off state, a blocking voltage dropsvertically between the first load electrode at the front side and thesecond load electrode on the back and drops laterally across atermination area between an active area in a central region of thedevice and a doped edge region that is formed along the lateral surfaceof the semiconductor die and that has the electric potential of thesecond load electrode. Typically, a junction termination extensionforming a pn junction with a drift zone shapes the lateral electricfield in the termination region in a way that avoids high electric fieldstrength along the front side of the semiconductor die.

There is a need for junction termination extensions that avoid highelectric field strength inter alia in semiconductor materials with lowdiffusion coefficients for dopant ions. There is also a need for methodsfor forming such junction termination extensions.

SUMMARY

The present disclosure relates to a semiconductor device including adrift zone formed in a semiconductor portion that includes a transitionsection in which a vertical extension of the semiconductor portiondecreases from a first vertical extension to a second verticalextension. A junction termination zone of a conductivity typecomplementary to a conductivity type of the drift zone is formed betweena first surface of the semiconductor portion and the drift zone. Thejunction termination zone includes a tapering portion in the transitionsection. In the tapering portion a vertical extension of the junctiontermination zone decreases from a maximum vertical extension to zerowithin a lateral width of at least twice the maximum vertical extension.

The present disclosure also relates to a semiconductor device includinga drift zone, which is formed in a semiconductor portion. Thesemiconductor portion includes a central area with a first verticalextension and a termination area. The termination area includes an edgesection with a second vertical extension and a transition section inwhich the vertical extension gradually decreases from the first verticalextension to the second vertical extension. A junction termination zonebetween a first surface of the semiconductor portion and the drift zonehas a conductivity type complementary to a conductivity type of thedrift zone and includes a tapering portion in the transition section.

The present disclosure further relates to a method of manufacturingsemiconductor devices. In a semiconductor substrate a doped region ofuniform vertical extension is formed at least in a portion of atermination area of a device region, wherein the termination areasurrounds a central area of the device region. An etch mask is formed ona substrate surface of the semiconductor substrate. The etch maskincludes an etch mask opening that exposes an edge section of thetermination area and that includes a tapering section in a transitionsection between the central area and the edge section. The semiconductorsubstrate is recessed by using a directional etch process to form, fromthe doped region, a junction termination zone that includes a taperingportion defined by the tapering section of the etch mask.

Further embodiments are described in the dependent claims. Those skilledin the art will recognize additional features and advantages uponreading the following detailed description and on viewing theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the present embodiments and are incorporated in andconstitute a part of this specification. The drawings illustrate thepresent embodiments and together with the description serve to explainprinciples of the embodiments. Further embodiments and intendedadvantages will be readily appreciated as they become better understoodby reference to the following detailed description.

FIG. 1 is a schematic vertical cross-sectional view of a portion of asemiconductor device with a junction termination zone that includes atapering portion in a tapering transition section of a semiconductorportion according to an embodiment.

FIG. 2A is a schematic diagram illustrating blocking voltagedistributions along a surface of semiconductor portions for discussingeffects of the embodiments.

FIG. 2B is a schematic diagram illustrating electric field strengthdistributions along a surface of semiconductor portions for discussingeffects of the embodiments.

FIG. 3 is a schematic vertical cross-sectional view of a portion of asemiconductor device according to an embodiment related to a bowedtapering portion of a junction termination zone.

FIG. 4 is a schematic vertical cross-sectional view of a portion of asemiconductor device according to an embodiment related to linearlytapering junction termination zones.

FIG. 5 is a schematic vertical cross-sectional view of a portion of asemiconductor device according to an embodiment combining a taperingjunction termination zone with a recessed drift zone.

FIG. 6 is a schematic vertical cross-sectional view of a portion of asemiconductor device according to an embodiment combining a taperingjunction termination zone with a field ring.

FIG. 7 is a schematic vertical cross-sectional view of a portion of asemiconductor device according to an embodiment combining a taperingjunction termination zone with a field ring and with a recessed driftzone.

FIG. 8A is a schematic plan view of a semiconductor portion of asemiconductor device according to an embodiment combining a taperingjunction termination zone with a central anode/base region.

FIG. 8B is a schematic cross-sectional view of the semiconductor portionof FIG. 8A along line B-B.

FIG. 9 is a schematic vertical cross-sectional view of a semiconductordevice according to an embodiment related to a power semiconductordiode.

FIG. 10 is a schematic vertical cross-sectional view of a semiconductordevice according to an embodiment related to a vertical powersemiconductor switch.

FIG. 11 is a simplified flowchart illustrating a method of forming atapering junction termination zone for a vertical power semiconductordevice according to an embodiment.

FIG. 12A is a schematic vertical cross-sectional view of a portion of asemiconductor substrate for illustrating a method of manufacturingsemiconductor devices with lateral junction termination zones includingtapering portions according to an embodiment based on gray scalelithography, after forming a doped region of uniform vertical extensionin a termination area.

FIG. 12B is a schematic vertical cross-sectional view of thesemiconductor substrate portion of FIG. 12A, after forming an etch masklayer.

FIG. 12C is a schematic vertical cross-sectional view of thesemiconductor substrate portion of FIG. 12B, after forming a gray scalemask with tapering sections by using gray scale lithography.

FIG. 12D is a schematic vertical cross-sectional view of thesemiconductor substrate portion of FIG. 12C, after forming an etch maskwith tapering sections.

FIG. 12E is a schematic vertical cross-sectional view of thesemiconductor substrate portion of FIG. 12D, after etching the dopedregion by using the etch mask of FIG. 12D.

FIG. 13A is a schematic vertical cross-sectional view of a portion of asemiconductor substrate for illustrating a method of manufacturingsemiconductor devices with junction termination zones including taperingportions according to an embodiment based on a reflow of a precursormask, after forming the precursor mask.

FIG. 13B is a schematic vertical cross-sectional view of thesemiconductor substrate portion of FIG. 13A, after a heat treatment fora reflow of the precursor mask.

FIG. 13C is a schematic vertical cross-sectional view of thesemiconductor substrate portion of FIG. 13B, after recessing a dopedlayer by using the etch mask of FIG. 13B.

FIG. 14A is a schematic vertical cross-sectional view of a portion of asemiconductor substrate for illustrating a method of manufacturingsemiconductor devices with junction termination zones including taperingportions according to an embodiment based on a multi-layer etch mask incombination with isotropic etching, after forming a support mask on amulti-layer mask stack.

FIG. 14B is a schematic vertical cross-sectional view of thesemiconductor substrate portion of FIG. 14A during an isotropic etchundercutting the support mask.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof and in which are shownby way of illustrations of specific embodiments. It is to be understoodthat other embodiments may be utilized and structural or logical changesmay be made without departing from the scope of the present disclosure.For example, features illustrated or described for one embodiment can beused on or in conjunction with other embodiments to yield yet a furtherembodiment. It is intended that the present disclosure includes suchmodifications and variations. The examples are described using specificlanguage, which should not be construed as limiting the scope of theappending claims. The drawings are not scaled and are for illustrativepurposes only. Corresponding elements are designated by the samereference signs in the different drawings if not stated otherwise.

The terms “having”, “containing”, “including”, “comprising” and the likeare open, and the terms indicate the presence of stated structures,elements or features but do not preclude additional elements orfeatures. The articles “a”, “an” and “the” are intended to include theplural as well as the singular, unless the context clearly indicatesotherwise.

The term “electrically connected” describes a permanent low-resistiveconnection between electrically connected elements, for example a directcontact between the concerned elements or a low-resistive connection viaa metal and/or heavily doped semiconductor material. The term“electrically coupled” includes that one or more intervening element(s)adapted for signal transmission may be between the electrically coupledelements, for example, elements that are controllable to temporarilyprovide a low-resistive connection in a first state and a high-resistiveelectric decoupling in a second state.

The Figures illustrate relative doping concentrations by indicating “−”or “+” next to the doping type “n” or “p”. For example, “n−” means adoping concentration which is lower than the doping concentration of an“n”-doping region while an “n+”-doping region has a higher dopingconcentration than an “n”-doping region. Doping regions of the samerelative doping concentration do not necessarily have the same absolutedoping concentration. For example, two different “n”-doping regions mayhave the same or different absolute doping concentrations.

FIG. 1 shows a portion of a semiconductor device 500 that may be or mayinclude a power semiconductor diode, an MPS (merged pin Schottky) diode,an IGFET (insulated gate field effect transistor), for example, anMOSFET (metal oxide semiconductor FET) in the usual meaning includingIGFETs with metal gates as well as IGFETs with gates from asemiconductor material, a JFET (junction field effect transistor), anIGBT (insulated gate bipolar transistor), or an MCD (MOS controlleddiode), by way of example.

The semiconductor device 500 includes a semiconductor portion 100 with afirst surface 101 at a front side and a second surface 102 on the backopposite to the first surface 101. In a central area 611 of thesemiconductor portion 100 the first surface 101 includes a planar mainsurface 1011 at a first distance to the second surface 102. In an edgesection 6192 of a termination area 619 that surrounds the central area611, the first surface 101 includes a planar recessed surface 1013 at asmaller, second distance to the second surface 102.

A transition section 6191 of the termination area 619 between thecentral area 611 and the edge section 6192 of the first surface 101includes a connection surface 1012 connecting the main surface 1011 andthe recessed surface 1013, such that in the transition section 6191 avertical extension of the semiconductor portion 100 gradually decreasesfrom a first vertical extension d1 equal to the first distance to asecond vertical extension d2 equal to the second distance.

The first vertical extension d1 between the main surface 1011 and thesecond surface 102 may be in a range of several μm to several hundredμm. The second vertical extension d2 is at most 2 μm smaller than thefirst vertical extension d1. A normal to the main surface 1011 defines avertical direction and directions parallel to the main surface 1011 arehorizontal directions which are also referred to as lateral directionsin the following.

The semiconductor portion 100 may be based on a single-crystallinesemiconductor material in which a diffusion coefficient for typicaldopant atoms is significantly lower than for arsenic, boron andphosphorous atoms in single-crystalline silicon. According to anembodiment, the semiconductor portion is of silicon carbide (SiC) andincludes a drift structure 130. The drift structure 130 includes a driftzone 135 of a first conductivity type, wherein the drift zone 135 mayextend across a complete horizontal cross-sectional plane of thesemiconductor portion 100 and accommodates the main portion of ablocking voltage applied between the first and second surfaces 101, 102.The drift structure 130 further includes a heavily doped base portion139 between the drift zone 135 and the second surface 102.

A conductivity type of the base portion 139 may be the same as that ofthe drift zone 135, may be the complementary conductivity type or thebase portion 139 may include doped zones of both conductivity typesextending from the drift zone 135 to the second surface 102. Along thesecond surface 102 a dopant concentration of the base portion 139 may besufficiently high to form a low-resistive contact, for example, an ohmiccontact, with a metal structure that may directly adjoin the secondsurface 102.

A junction termination zone 170 between the first surface 101 and thedrift structure 130, e.g., between the first surface 101 and the driftzone 135 forms a first pn junction pn1 with the drift zone 135.

The first pn junction pn1 may include horizontal sections parallel tothe main surface 1011 and vertical sections orthogonal to the mainsurface 1011. According to an embodiment the first pn junction pn1 maybe completely planar and may extend in one single geometrical planeparallel to the main surface 1011. A maximum vertical extension v1 ofthe junction termination zone 170 may be equal to a difference Δd=d1−d2between the first distance d1 and the second distance d2, may be smallerthan Δd. The maximum thickness v1 may be in a range from 200 nm to 2 μm,for example in a range from 400 nm to 1000 nm.

A mean dopant concentration in the junction termination zone 170 may beuniform along the lateral direction. For example, a mean dopantconcentration in the junction termination zone 170 may be in a rangefrom 1E16 cm⁻³ to 1E18 cm⁻³, for example, in a range from 5E16 cm⁻³ to5E17 cm⁻³. The vertical dopant profile of the junction termination zone170 may be undulated with two or more local maxima and one or more localminima. Alternatively, the vertical dopant profile of the terminationzone 170 may be box-shaped, i.e., approximately uniform and withoutundulation, wherein, for example, an ion beam used for the implant maypass an energy filter before reaching the substrate.

In the transition section 6191 the junction termination zone 170includes a tapering portion 172 with a lateral width w1, wherein avertical extension of the junction termination zone 170 and the verticalextension of the semiconductor portion 100 decrease in the same way andby the same amount. In other words, along the tapering portion 172 thevertical extension of the tapering portion 172 changes by the sameamount as the vertical extension of the semiconductor portion 100.

In the tapering portion 172 the vertical extension of the junctiontermination zone 170 decreases from the maximum thickness v1 to zero,wherein a ratio w1:v1 of the lateral width w1 to the maximum verticalextension v1 is at least 2:1, for example, at least 3:1, at least 5:1,or at least 10:1. According to an embodiment the ratio w1:v1 is at least20:1. The junction termination zone 170 may further include anon-tapering portion 171 with a thickness equal to the maximum thicknessv1 and laterally adjoining the tapering portion 172.

A vertical cross-sectional line of the connection surface 1012 may becompletely or partially crooked, curved, concavely or convexly bowed,and may include straight sections parallel to or tilted to the mainsurface 1011. According to an embodiment the vertical extension of thetapering portion 172 may decrease monotonically. For example, thevertical extension of the tapering portion 172 may decrease strictlymonotonic, e.g., linearly.

As a result, a total amount of dopant ions in the junction terminationzone 170 steadily decreases. The blocking voltage drops across acomparatively large distance of several micrometers and the maximumlateral electric field strength is low compared to junction terminationextensions with a steep lateral termination.

Without tapering portion 172, in a semiconductor device based on asemiconductor material with low diffusion coefficients for dopant ions,the total amount of dopant ions in the junction termination zone 170decreases abruptly, the blocking voltage drops across a short distanceand the maximum electric field strength is high. In semiconductordevices based on semiconductor materials with high diffusioncoefficients for dopant atoms, a thermally induced diffusion of thedopants can smooth pn junctions in a way that the pn junctions smoothlyaccommodate the blocking voltage across a comparatively large lateraldistance such that the maximum electric field strength in thetermination area is comparatively low.

FIG. 2A shows the effect of a junction termination zone with awedge-shaped tapering portion according to an embodiment related to asilicon carbide diode with a blocking capability of 650 V.

Line 401 plots the lateral potential along the first surface for acomparative example without tapering junction termination. Line 402plots the lateral potential along the first surface 101 for anembodiment with a tapering junction termination having a lateral widthw1 of 10 μm. The tapering portion significantly reduces steepness of thelateral potential.

FIG. 2B shows the lateral electric field strength along the firstsurface, wherein line 403 refers to the comparative example and line 404refers to the embodiment with the tapering portion having a lateralwidth w1 of 10 μm. The maximum electric field strength is at most 30% ofthe maximum electric field strength in the comparative example.

In FIG. 3 a vertical extension of the tapering portion 172 decreasesstrictly monotonic across the complete lateral tapering width w1 in anon-linear manner.

FIG. 4 shows a wedge-shaped tapering portion 172 in which the verticalextension of the junction termination zone 170 linearly decreases fromthe maximum thickness v1 to zero.

A side surface 103 connects the first surface 101 and the second surface102 and may be vertical with respect to the second surface 102. Alateral width w3 of the edge section 6192 is equal to a lateral distancew4 between the junction termination zone 170 and the side surface 103and may be at least as large as a thickness d3 of the drift zone 135, orat least twice d3. For example, the lateral width w3 of the edge section6192 may be at least 3 μm, for example, at least 4.5 μm for a siliconcarbide device with a blocking voltage capability of 650 V and may be atleast 8 μm, for example, at least 9.5 μm for a silicon carbide devicewith a blocking voltage capability of 1200 V. A lateral width w2 of thetransition section 6191 is equal to a lateral width w1 of the taperingportion 172 of the junction termination zone 170.

In FIG. 5 the difference Δd between the first distance d1 and the seconddistance d2 is greater than the maximum vertical extension v1 of thejunction termination zone 170. A vertical recess v3 between the first pnjunction pn1 and the recessed surface 1013 is at most equal to themaximal vertical extension v1 of the junction termination zone 170. Alateral width w2 of the transition section 6191 is greater than alateral width w1 of the tapering portion 172 of the junction terminationzone 170. The lateral distance w4 between the junction termination zone170 and the side surface 103 may be at least the thickness d3 of thedrift zone 135, or at least twice d3.

In the previous embodiments, the edge section 6192 of the semiconductorportion 100 between the junction termination zone 170 and the sidesurface 103 is devoid of further structures such as doped regions.According to other embodiments, the semiconductor portion 100 mayinclude further doped regions, for example, one or more field ringsbetween the junction termination zone 170 and the side surface 103.

In FIG. 6 the edge section 6192 includes a field ring 190 that includesa doped field ring region 191 forming a second pn junction pn2 with thedrift zone 135 in a section of the semiconductor portion 100 between thejunction termination zone 170 and the side surface 103. The doped fieldring region 191 may float. The field ring region 191 may protrude fromthe recessed surface 1013, wherein a vertical extension v2 of the fieldring region 191 may be approximately the same as the maximum verticalextension v1 of the junction termination zone 170. A mean net dopantconcentration in the field ring region 191 may be within the same orderof magnitude as in the junction termination zone 170. According to anembodiment the mean net dopant concentration in the field ring region191 may be the same as in the junction termination zone 170.

The second pn junction pn2 may be coplanar with at least an outersection of the first pn junction pn1. According to an embodiment, thesecond pn junction pn2 may be coplanar with the recessed surface 1013 asillustrated in FIG. 6.

In the semiconductor device 500 of FIG. 7 the second pn junction pn2 iscoplanar with the first pn junction pn1 and has a greater distance tothe second surface 102 as the recessed surface 1013.

FIGS. 8A and 8B show a semiconductor portion 100 of a semiconductordevice 500 with a central area 611 and a termination area 619, whereinthe termination area 619 surrounds the central area 611 and separatesthe central area 611 from a side surface 103.

The central area 611 includes an active area of the semiconductor device500, wherein the active area may include an anode/body region 120forming a main pn junction pnx with a drift structure 130 as describedabove. The anode/body region 120 may be an anode region of a powersemiconductor diode, may include the anode zones of pn-diode cells of anMPS or may include body regions of transistor cells of a powersemiconductor switch, e.g., an IGFET or an IGBT. In addition, thecentral area 611 may include interconnection structures, e.g., gaterunners for electrically connecting gate electrodes of transistor cellsin the central area 611.

The drift structure 130 includes a drift zone 135 between the anode/bodyregion 120 at the front side and the second surface 102 on the back,wherein the anode/body region 120 forms the main pn junction pnx withthe drift zone 135.

A junction termination zone 170 is laterally connected with theanode/body region 120 and may laterally surround the anode/body region120 in the termination area 619. The junction termination zone 170 formsa first pn junction pn1 with the drift zone 135, wherein the first pnjunction pn1 is parallel to the horizontal plane or includes sectionsparallel to the horizontal plane. The junction termination zone 170includes a tapering portion 172 in which the vertical extension of thejunction termination zone 170 decreases from a maximum verticalextension to a minimum vertical extension, which may be zero. The changeof the vertical extension of the tapering portion 172 may correspond toa change of the vertical extension of the semiconductor portion 100 bythe same amount.

The tapering portion 172 extends between a connection surface 1012 ofthe first surface 101 and the drift structure 130, wherein theconnection surface 1012 is not parallel to the horizontal plane.

FIGS. 9 and 10 refer to vertical cross-sections of semiconductor devices500, wherein vertical cross-sections orthogonal to the illustratedcross-section may widely correspond to or may be qualitatively identicalto the illustrated cross-sections.

In FIG. 9 the semiconductor device 500 is a power semiconductor diodewith a semiconductor portion 100 of a single-crystalline semiconductormaterial in which a diffusion coefficient for typical dopant atoms issignificantly lower than for arsenic, boron and phosphorous atoms insingle-crystalline silicon, e.g., silicon carbide (SiC). For example,the semiconductor portion 100 may be based on 4H—SiC (SiC of the4H-polytype), 2H—SiC, 6H—SiC or 15R—SiC. A first surface 101 of thesemiconductor portion 100 at the front side includes a mainly planarmain surface 1011 in a central area, a mainly planar recessed surface1013 parallel to the main surface 1011 along a side surface 103 and atilted connection surface 1012 connecting the main surface 1011 and therecessed surface 1013. On the back, an opposite second surface 102 isparallel to the main surface 1011 and to the recessed surface 1013.

A first vertical extension d1 of the semiconductor portion 100 betweenthe main surface 1011 and the second surface 102 in a central area maybe in a range of several μm to several hundred μm. A drift structure 130directly adjoins the second surface 102. The drift structure 130 mayinclude a lightly doped drift zone 135 as well as a heavily doped baseportion 139 between the drift zone 135 and the second surface 102,wherein the base portion 139 has the same conductivity type as the driftzone 135.

The drift structure 130 may be electrically connected or coupled to asecond load electrode 320 through a low-resistive contact. For example,a dopant concentration in the base portion 139 along the second surface102 is sufficiently high to form a low-resistive ohmic contact with thesecond load electrode 320 that directly adjoins the second surface 102.The second load electrode 320 forms or is electrically connected orcoupled to a cathode terminal K of the semiconductor diode.

A net dopant concentration in the drift zone 135 may be in a range from1E14 cm⁻³ to 3E16 cm⁻³ in case the semiconductor portion 100 is based onsilicon carbide. The drift structure 130 may include further dopedregions between the drift zone 135 and the first surface 101 and betweenthe drift zone 135 and the second surface 102.

In the central area an anode region 122 forms a main pn junction pnxwith the drift structure 130, for example, with the drift zone 135. Themain pn junction pnx may be parallel to the main surface 1011. A firstload electrode 310 directly adjoins the anode region 122 and may form ormay be electrically connected or coupled to an anode terminal A.

A transition section, in which the thickness of the semiconductorportion 100 gradually decreases from the first vertical extension d1 inthe central area to a second vertical extension d2 along the sidesurface 103, includes a tapering portion 172 of a junction terminationzone 170, wherein the vertical extension of the tapering portion 172decreases in the same way and by the same amount as the verticalextension of the semiconductor portion 100. A mean net dopantconcentration in the junction termination zone 170 may be lower than inthe anode region 122.

A dielectric layer 210 may separate the tapering portion 172 or at leasta section of the tapering portion 172 from the first load electrode 310.For example, the dielectric layer 210 may cover the tapering portion 172or at least cover a section of the tapering portion 172 and may overlapon the first load electrode 310 and may overlap on the surface 1013.

FIG. 10 shows a semiconductor device 500 including transistor cells TC.The semiconductor device 500 may be, for example, an IGFET, a JFET, anIGBT or an MCD. Regarding details of the semiconductor portion 100, thedrift structure 130 and the junction termination zone 170, reference ismade to the description of the semiconductor diode in FIG. 9.

Instead of an anode region, the semiconductor device 500 of FIG. 10includes transistor cells TC, wherein in each transistor cell TC a bodyregion 125 separates a source region from the drift structure 130. Thebody regions 125 form first transistor pn junctions, which correspond tothe main pn junctions pnx of FIG. 9, with the drift structure 130, e.g.,with the drift zone 135. The body region 125 forms second transistor pnjunctions with the source zones.

A first load electrode 310 electrically connected to the body regions125 and the source regions of the transistor cells TC may form or may beelectrically connected or coupled to a first load terminal L1, which maybe an anode terminal of an MCD, a source terminal of an IGFET or JFET,or an emitter terminal of an IGBT.

A second load electrode 320 electrically connected to the base portion139 may form or may be electrically connected or coupled to a secondload terminal L2, which may be a cathode terminal of an MCD, a drainterminal of an IGFET or JFET, or a collector terminal of an IGBT.

The transistor cells TC may be transistor cells with planar gateelectrodes or with trench gate electrodes, wherein the trench gateelectrodes may control a lateral channel or a vertical channel.According to an embodiment, the transistor cells TC are n-channel IGFETcells of the enhancement type with p-doped body regions 125, n-dopedsource zones and an n-doped drift zone 135. Other embodiments mayconcern p-channel IGFET cells of the enhancement type, depletion-typeIGFET cells, normally-on JFET cells, or normally-off JFET cells.

According to FIG. 11 a method of manufacturing a silicon carbide devicewith a junction termination extension includes forming a doped region ofuniform vertical extension in a termination area of a device region of asemiconductor substrate of silicon carbide, wherein the termination areasurrounds a central area of the device region (902). An etch mask isformed on a substrate surface of the semiconductor substrate (904),wherein the etch mask includes an etch mask opening exposing a portionof the termination area and wherein the etch mask includes tapering masksections in which a thickness of the etch mask decreases from a maximumthickness to zero within a lateral distance of at least twice themaximum thickness.

The doped region is etched (906) in a directional etch process to form ajunction termination zone including a tapering portion defined by thetapering section of the etch mask.

FIGS. 12A to 12E refer to an embodiment using gray scale lithography forforming an etch mask with tapering mask sections.

A doped region 770 of uniform vertical extension v1 is formed byintroducing dopants into a semiconductor substrate 700, e.g., by ionimplantation through a substrate surface 701. The semiconductorsubstrate 700 may be a silicon carbide substrate, e.g., of 4H—SiC or6H—SiC and may include a base substrate 705, which may be a siliconcarbide slice obtained from a silicon carbide ingot by sawing, by way ofexample. The base substrate 705 may be heavily doped, for exampleheavily n-doped. An epitaxy process may form a drift layer structure 730on a process surface of the base substrate 705. The semiconductorsubstrate 700 may include further doped regions, for example, one ormore p-doped regions laterally adjoining the doped region 770 andforming an anode region of a semiconductor diode or body regions oftransistor cells.

FIG. 12A shows the doped region 770 that may be formed exclusively in atermination area 619 of a device region 610 or that may extend into akerf region 690 between neighboring device regions 610, wherein thedevice regions 610 correspond to kernels of semiconductor dies of singlesemiconductor devices obtained from the semiconductor substrate 700 at alater stage and wherein the termination areas 619 surround central areas611 that include active device areas such as transistor cell arrays oranode regions.

A mean net dopant concentration in the doped region 770 may be constantalong the lateral direction. The vertical extension v1 of the dopedregion 770 may be constant such that a pn junction between the dopedregion 770 and the drift layer structure 730 is planar and extends in asingle geometric plane parallel to the substrate surface 701. Accordingto other embodiments, forming the doped region 770 may include two ormore implants with different vertical extensions, such that the first pnjunction may include several planar sections in different distances tothe substrate surface 701. An etch mask layer 410 may be deposited onthe substrate surface 701.

FIG. 12B shows the etch mask layer 410, which may include one singlelayer or which may include a layer stack including sub layers ofdifferent materials. The etch mask layer 410 may include or consist of alayer of silicon oxide or silicate glass, for example, a layer of PSG(phosphorous silicate glass). According to other embodiments, the etchmask layer 410 includes silicon oxide formed by deposition of TEOS(tetraethylorthosilane).

A gray scale mask layer is deposited on the etch mask layer 410 andpatterned by gray scale lithography to form a gray scale mask 421.

FIG. 12C shows the gray scale mask 421, which may include a photoresistmaterial. The gray scale mask 421 may cover central areas 611 of thedevice regions 610 at uniform thickness. A gray scale mask opening 425exposes a section of the etch mask layer 410 in the kerf region 690 andin portions of the device regions 610 directly adjoining the kerf region690. In tapering sections 427 along the gray scale mask opening 425 thethickness of the gray scale mask 421 gradually decreases from a maximumthickness to zero.

Using the gray scale mask 421 as etch mask, the etch mask layer 410 islocally recessed by an etch process that consumes both the material ofthe etch mask layer 410 and the material of the gray scale mask 421. Therecess rates for the etch mask layer 410 and the gray scale mask 421 maybe selected such that the gray scale mask 421 is completely consumedshortly after the doped region 770 is exposed, wherein the taperingsection 427 of the gray scale mask 421 is imaged into tapering masksections 417 of an etch mask 411 formed from a remnant portion of theetch mask layer 410.

FIG. 12D shows the etch mask 411 covering the central areas 611 atapproximately uniform thickness and including an etch mask opening 415exposing the doped region 770 in the kerf region 690 and in edgesections 6192 directly adjoining the kerf region 690. The tapering masksections 417 cover inner transition sections 6191 of termination areas619 directly adjoining the central areas 611.

A further etch process transfers the pattern of the etch mask 411 intothe semiconductor substrate 700, wherein the etch process recesses boththe etch mask 411 and the semiconductor substrate 700. The etch processis selected such that the etch mask 411 is completely removed shortlyafter the etching has reached the pn junction between the doped region770 and the drift layer structure 730. The etch process may include ahigh physical portion. For example, the etch process may include ionbeam milling.

FIG. 12E shows the recessed semiconductor substrate 700 with the dopedregion 770 as illustrated in FIG. 12D completely removed both in thekerf region 690 and in edge sections 6192 of the termination areas 619directly adjoining the kerf region 690. Transition sections 6191 of thetermination areas 619 include wedge-shaped junction termination zones170 forming first pn junctions pn1 parallel to unrecessed sections 7011of the substrate surface 701. The substrate surface 701 further includesrecessed sections 7013 and connection sections 7012 connecting theunrecessed sections 7011 with the recessed sections 7013. Etching maystop at the plane of the pn junction or below the pn junction such thatthe tapering portion 172 of the junction termination zone 170 isnarrower than the transition section 6191, in which the verticalextension of the semiconductor portion 100 decreases.

The process gets along without an implantation process using aphotoresist material as implant mask with tapering sections and avoidsformation of implant-induced resist residuals, the removal of which maybe a complex task, because the implantation process may convert thinsections of a photoresist layer in residuals, which are typically hardto remove.

According to another embodiment, deposition and patterning of the etchmask layer 410 are omitted and the gray scale mask 421 is directlyformed on the substrate surface 701.

FIGS. 13A to 13C illustrate a method of forming an etch mask withtapering portions on the basis of a reflow process. As regards formationof a doped region 770 in a semiconductor substrate 700 reference is madeto the description of FIG. 12A.

A precursor mask layer is deposited on the substrate surface 701 andpatterned by photolithography to form a precursor mask 431 covering thecentral areas 611 of the device regions 610 and including precursor maskopenings 435 exposing sections of a substrate surface 701 in a kerfregion 690 and in outer sections of the termination areas 619 directlyadjoining the kerf region 690.

FIG. 13A shows the precursor mask 431 including a precursor mask opening435 with vertical sidewalls within the termination areas 619. Theprecursor mask 431 covers central areas 611 of the device regions 610and exposes both the kerf region 690 and outer sections of thetermination areas 619 directly adjoining the kerf region 690. Theprecursor mask 431 may include a material with well-defined reflowproperties at comparatively low temperatures, e.g., below 800° C. suchas doped silicate glass, e.g., PSG, BSG (boron silicate glass), BPSG(boron phosphorous silicate glass), or FSG (fluorine silicate glass).

The semiconductor substrate 700 is subjected to a heat treatment at atemperature at which the precursor mask 431 starts to reflow and thesteep sidewalls of the precursor mask opening 435 start to degrade. Theheat treatment is terminated, when a region in which the thickness ofthe precursor mask material is not uniform, reaches a target widthcorresponding to a target width of the transition sections 6191.

FIG. 13B shows an etch mask 411 formed by reflow of the precursor mask431 of FIG. 13A. The etch mask 411 includes etch mask openings 415exposing edge sections 6192 of the termination area 619 as well as thekerf region 690. A suitable etch process images the contour of the etchmask 411 into the semiconductor substrate 700 as described with respectto FIG. 12E.

FIG. 13C shows the semiconductor substrate 700 after removal of the etchmask 411 of FIG. 13B. The substrate surface 701 includes unrecessedsections 7011 in the central areas 611, a recessed section 7013 in boththe kerf region 690 and the edge sections 6192 of the termination areas619. In the transition sections 6191 of the termination areas 619,curved connection sections 7012 of the substrate surface 701 connect theunrecessed sections 7011 with the recessed section 7013.

The etch process forms, from the doped region 770 of FIG. 13B, separatedjunction termination zones 170 laterally spaced from one another.Directly below the connection section 7012 a vertical extension of thejunction termination zones 170 decreases from a maximum verticalextension v1 to zero.

FIGS. 14A to 14B relate to a method forming a suitable etch mask fortapering junction termination zones on the basis of a multi-layer etchmask. A doped region 770 may be formed in a semiconductor substrate 700as described with reference to FIG. 12A.

A multi-layer stack 440 including at least a mask base layer 447 and amask top layer 448 is formed on the substrate surface 701. The mask baselayer 447 and the mask top layer 448 may be formed from materials withdifferent etching properties.

According to an embodiment, a starting layer, for example, a siliconoxide is deposited and impurities, for example, boron or phosphorousatoms, are implanted into a top section of starting layer, wherein thetop section forms the mask top layer 448 and the unmodified section ofthe starting layer forms the mask base layer 447. A support mask layeris deposited and patterned by photolithography to form a support mask451 covering the central areas 611 of the device regions 610 andincluding support mask openings 455 above the kerf region 690 and aboveedge sections 6192 of the termination areas 619 directly adjoining thekerf region 690.

FIG. 14A shows the support mask 451 on the multi-layer stack 440,wherein the support mask 451 covers portions of the multi-layer stack440 in the vertical projection of the central areas 611 of the deviceregions 610 and wherein the support mask 451 includes support maskopenings 455 directly above the kerf region 690 and directly aboveadjoining edge sections 6192 of the termination areas 619.

A wet etch that recesses the mask top layer 448 at a higher rate thanthe mask base layer 447 forms etch mask openings 415 in the verticalprojection of the support mask openings 455.

FIG. 14B shows the effect of the wet etch. Due to the higher etch rateof the mask top layer 448, the wet etch undercuts the support mask 451at a higher rate than the wet etch recesses the mask base layer 447. Thewet etch gradually exposes an upper edge of the mask base layer 447below the support mask 451. At the end of the wet etch, remnant portionsof the mask base layer 447 and the mask top layer 448 form an etch maskwith wedge-shaped tapering mask sections 417.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

What is claimed is:
 1. A semiconductor device, comprising: a drift zoneformed in a semiconductor portion that comprises a transition section inwhich a vertical extension of the semiconductor portion decreases from afirst vertical extension to a second vertical extension; and a junctiontermination zone of a conductivity type complementary to a conductivitytype of the drift zone, wherein the junction termination zone is betweena first surface of the semiconductor portion and the drift zone andcomprises a tapering portion in the transition section, wherein in thetapering portion a vertical extension of the junction termination zonedecreases from a maximum vertical extension to zero within a lateralwidth of at least twice the maximum vertical extension.
 2. Thesemiconductor device of claim 1, wherein the junction termination zoneforms a planar first pn junction with a drift structure comprising thedrift zone.
 3. The semiconductor device of claim 1, wherein the junctiontermination zone forms a first pn junction with the drift zone.
 4. Thesemiconductor device of claim 1, wherein the transition section isformed within a termination area between a central area and a sidesurface of the semiconductor portion, and wherein the central areacomprises transistor cells or an anode region in electric contact with afirst load electrode.
 5. The semiconductor device of claim 4, whereinthe vertical extension of the junction termination zone decreases withincreasing distance to the central area.
 6. The semiconductor device ofclaim 4, wherein the transition section surrounds the central area. 7.The semiconductor device of claim 1, wherein the semiconductor portioncomprises a central area with the first vertical extension and atermination area that comprises the transition section and an edgesection with the second vertical extension, and wherein the edge sectionis between the transition section and a side surface of thesemiconductor portion.
 8. The semiconductor device of claim 7, wherein alateral distance between the junction termination zone and the sidesurface is at least as large as a thickness of a drift zone formed inthe drift structure.
 9. The semiconductor device of claim 1, wherein thejunction termination zone laterally adjoins and surrounds an anode/bodyregion forming a main pn junction with the drift structure.
 10. Thesemiconductor device of claim 1, wherein the vertical extension of thejunction termination zone decreases monotonically.
 11. The semiconductordevice of claim 1, wherein the vertical extension of the junctiontermination zone decreases linearly.
 12. The semiconductor device ofclaim 1, wherein a lateral width of the transition section is equal to alateral width of the tapering portion of the junction termination zone.13. The semiconductor device of claim 1, wherein a lateral width of thetransition section is greater than a lateral width of the taperingportion of the junction termination zone.
 14. The semiconductor deviceof claim 1, further comprising: a field ring comprising a field ringregion protruding from a first surface of the semiconductor portionbetween the junction termination zone and a side surface of thesemiconductor portion, wherein the field ring region forms a second pnjunction with the drift structure.
 15. The semiconductor device of claim1, wherein the semiconductor portion comprises silicon carbide.
 16. Asemiconductor device, comprising: a drift zone formed in a semiconductorportion that comprises a central area with a first vertical extensionand a termination area, the termination area comprising an edge sectionwith a second vertical extension and a transition section in which thevertical extension gradually decreases from the first vertical extensionto the second vertical extension; and a junction termination zonebetween a first surface of the semiconductor portion and the drift zone,the junction termination zone having a conductivity type complementaryto a conductivity type of the drift zone and comprising a taperingportion in the transition section, wherein in the tapering portion avertical extension of the junction termination zone gradually decreasesfrom a maximum vertical extension to zero within a lateral distance ofat least twice the maximum vertical extension.